Will add-ons from Blender 2.79 work or be compatible with Blender 2.8?
No, most 2.7# addons will not be compatible with 2.8# versions of Blender, and will require explicit porting and updating.

The Python API has changed in ways that break compatibility. It has been improved and expanded to allow further integration, more features, and better workflows and these would make old addons not work in newer versions.

There is a Blender Wiki article documenting the new Python API changes and explaining how to port old addons.